1. Field of the Invention
The present invention relates to a pixel structure, and more particularly to a pixel unit with improved viewing angles of a liquid crystal display.
2. Background of the Invention
Liquid crystal displays have been widely applied in electrical products, such as computer monitors and TV monitors, for a long time. To provide a wider viewing range, Fujitsu commercialized a multi-domain vertically aligned liquid crystal display (MVA-LCD) in 1997. MVA has almost perfect viewing angle characteristics. However, a notable weak point is that the skin color of Asian people (light orange or pink) appears whitish from an oblique viewing direction.
The solid line in FIG. 1 shows the transmittance-voltage (T-V) characteristics of the MVA in the normal direction. The vertical axis is the transmittance rate. The horizontal axis is the applied voltage. When the applied voltage increases, the transmittance rate curve 101 in the normal direction also increases. The transmittance changes monotonically as the applied voltage increases. However, in the oblique direction, the transmittance rate curve 102 winds and the various gray scales become the same. Especially in the region 100, the transmittance changes decrease as the applied voltage increases. This is the main reason that the skin color of Asian people appears whitish from an oblique viewing direction.
A method is provided to improve this foregoing problem. This method combines two different T-V characteristics. The dashed line 201 in FIG. 2 shows the original T-V characteristics in the oblique viewing direction. The dashed line 202 in FIG. 2 shows other T-V characteristics with a higher threshold voltage. By optimizing the threshold voltage and the maximum transmittance of these two lines, monotonic characteristics can be achieved, as shown by the solid line 203 in FIG. 2. According to the typical method, each pixel is divided into two areas. One area has the original threshold voltage and the other area has a higher one.
There is a residual image problem in the typical method. According to the typical method, each pixel unit includes a plurality of sub-pixels. Each sub-pixel may generate different voltage changes after the voltage applied to the pixel unit is removed. The different voltage change may generate different data voltage in two adjacent frames when corresponding to a common electrode, which may affect the image quantity.
Therefore, it is also an objective to improve the image quality.